The function of the Transgenic Core Facility is to provide the investigators with the ability to inactivate and/or modify genes in vivo and thus to expedite the pursuit of the goals of the Program Project. The construction of standard transgenic mice and the targeted inactivation of genes in embryonic stem cells and subsequent production of chimeric animals are technically sophisticated and labor intensive procedures. Furthermore they involve a substantial investment in equipment. Consequently, the work and equipment should not be duplicated in the several laboratories within the Program Project. The centralized Transgenic Core Facility proposed here will provide transgenic animals more efficiently and reliably. Projects 3 and 4 within the Program will generate transgenic mouse models with reconstituted CD40 signaling in CD40 -/- knock-out mice or with disrupted signaling elements in the CD40 signaling pathway in normal mice, The B cells from such animals will be used in Projects 1, 2 and 3 to assess their utility in therapeutic protocols designed to induce tolerance to engraftment. The following core functions are proposed: 1) centralized housing of embryo donors and recipients 2) culturing, transformation, and analysis of embryonic stem cells 3) harvesting, microinjection, and re-implantation of embryos 4) delivery, fostering, and weaning of potentially transgenic animals 5) screening for gene integration 6) breeding and maintenance 7) cryopreservation of embryos